Sagittarius A*

At the center of the Milky Way lies a mysterious object of enormous mass, generally believed to be a black hole. All the gas, dust and stars in the galaxy, including our solar system, orbit this central point, called Sagittarius A* (pronounced "A-star"). This area is obscured by huge clouds of dust, but it can be seen in the radio wavelengths, which easily pass through the clutter (VLA image above).

We can measure how fast our solar system is moving by observing how Sgr A* changes in relation to a very distant object, outside the Milky Way. This is comparable to riding a carousel and watching the scenery glide behind the center support. It is neither the background nor the carousel support that actually move, but the rotation of the observer which causes this effect. In this case, the movement of the sun causes the apparent position of Sgr A* to change.

The experiment uses the quasars 1745-283 and 1748-291 (named for their locations in our sky) as fixed points, and compares the apparent change in position of Sgr A* to those points (see this image, left panel). After observations over the course of two years, the results (right panel) indicate that the Sun takes 226 million years to circle the galaxy. The experiment also lends credence to the theory that Sgr A* is in fact a supermassive black hole, because the mass and density required to cause this movement are far beyond that of common objects, such as stars or pulsars.